The invention is based on a method for diagnosis on a load path in a vehicle and on a circuit arrangement for diagnosis on a load.
Electronic controllers are known generally for controlling a wide variety of functions and devices and are increasingly being used in the automotive sector for the purpose of controlling a wide variety of functions and/or applications in a motor vehicle. An important class of such functions and/or applications has what are known as “low-side load circuits”, wherein a first connection of such load circuits is permanently connected to a negative pole of a vehicle battery or to the vehicle bodywork and a second connection of the load circuits is switched by an electronic controller. By way of example, the load circuits comprise solenoid valves, electromechanical relays, electric motors, etc. Typically, these load circuits are switched by means of a load switch, which is also called “high-side switch”, the first connection of which is permanently connected to a positive pole of the vehicle battery and the second connection of which is connected to the second connection of the load circuit. In the automotive sector, it is customary to perform a diagnosis before the load circuit is activated. The diagnosis is important particularly when the load circuit is arranged outside the electric controller and is connected to the latter by means of electrical lines. By way of example, the diagnosis is meant to recognize shorted or broken lines, nonconnected cables, etc. These cases of error need to be distinguished from the normal circuit situation. A misdiagnosis in the case of a shorted load circuit can result in triggered or blown fuses, malfunctions in the electronic controller or thermal damage, for example. Depending on the specific function and/or application, the diagnosis on the load circuit may be a simple or complex task. One frequently used approach involves a high-value pull-up resistor being looped into the line and the voltage being monitored. Depending on the voltage which is sensed by the electronic controller, it is possible to determine the state of the load circuit. If the sensed voltage exceeds a prescribed threshold value when the load switch is open, for example, it is possible to infer that the load circuit is not connected. Low-resistance load circuits are very difficult for diagnosis, since the nominal value of the load resistance corresponds almost to a nonreactive resistance value in the case of a typical short circuit. The aforementioned approach is therefore unable to distinguish between a shorted line and a normal operating state.